With increasing penetration of wind power generation, the requirements for the connection of wind power plants (WPPs) to the electrical grid are defined by new and emerging grid connection codes. The grid connection requirements vary in different parts of the world, but they share common aims, like to permit the development, maintenance, and operation of a coordinated, reliable, and economical transmission or distribution system.
The new requirements generally demand that WPPs provide ancillary services to support the network in which they are connected. WPPs differ from other generation sources—therefore they are particular in certain aspects of their control.
The main differences with synchronous generators (SGs) are as follows:
the energy source fluctuates and is unpredictable,
WPPs are highly modular and composed by a large number of generation units, thus introducing communication delays when sending the operational set points, and these could be in the range of hundredths of milliseconds,
the SGs provide the reactive power feed directly to the transmission system through their unit transformer which provides a more efficient voltage regulation since the reactive power is not transported over a long distance and through several transformers, and
wind turbine generators (WTGs) have bigger limitations in their reactive power and current capability due to the size of their electronic power converters.
Therefore, replacing traditional power plants, including their control characteristics during periods of strong wind could be a concern. Grid operators are solving this challenge by means of redacting specific sections in the grid codes for WPP performance.
Normally, the requirements of the utilities can be solved by the wind turbines and the plant controller. The wind power plant control can take place at wind turbine level and/or at plant level. The WPP controller can range from simply passing on references to the wind turbine control systems to performing the majority of closed-loop controls itself.
As voltage control is actuated by reactive power from the wind turbines it is possible to affirm that the voltage control design will cover an inner reactive power/voltage control loop, located at turbine level combined with an outer voltage control loop located at the point of common coupling (PCC) where it is intended to control the voltage.
However, at high active power levels it may occur that wind turbines, in particular doubly-fed induction generators, are not able to provide sufficient reactive power for voltage control. In such situation additional reactive power sources may be activated in order to ensure that a wind power plant meets the grid code requirement.
It may be seen as an object of embodiments of the present invention to provide a method and a control arrangement for controlling a reactive power source, in particular, at high active power levels.